1.0 Field of the Invention
The present invention relates to resin compositions adapted to be applied to fibrous substrates so as to improve their physical properties, notably the dry strength of paper substrates.
It is desirable to improve the dry strength properties of paper and paperboard. The dry strength properties of paper and paperboard are governed by their structural elements such as fiber strength and fiber-fiber bonding, as well as other factors including fiber length, sheet formation, sheet flaws, etc. In order to improve the internal dry strength properties of paper and paperboard, chemical additives are typically added thereto. The primary purpose of such chemical additives is to augment fiber-fiber bonding in the paper sheet. Interfiber hydrogen bonds form as a natural result of drying a wet paper web. Although the degree of interfiber bonding can be controlled through mechanical refining such as beating, nonfibrous additives that can enhance interfiber bonding are constantly sought.
2.0 Discussion of Related Art
In an effort to improve the dry strength properties of paper, synthetic resins were first used in the early 1940's and 1950's, such as for example, acrylamide polymers. Polymers of polyacrylamide were found to possess unique properties that made them especially effective as dry-strength resins. While other types of synthetic dry-strength resins are reported in the literature, commercial products are primarily based on acrylamide.
There are many benefits to be gained from the use of dry-strength additives. Refining can be reduced while maintaining paper strength, resulting in energy savings and increased production. Strength properties can be maintained while substituting a lower-strength, lower-cost furnish. In addition, dry strength can be increased without a corresponding increase in apparent density, as would be the case with increased refining.
In addition to the afore-mentioned acrylamide polymers, various other compositions have been reported as providing dry-strength properties. Most of these compositions can be classified as being cationic non-acrylamide-containing polymers, for example, vinyl pyridine and copolymers thereof, vinyl sulfonium polymers, polyacrylic hydrazide, cationic acrylate copolymers, polyvinylimidazoline, and condensation polymers of polyamines, ketones and aldehydes. Other dry-strength additives include water-soluble, hydrophilic natural polymers such as starch, vegetable gums, and carboxymethyl cellulose. However, there is a continuing need to provide dry-strength paper additives having improved properties.
3.0 Description of the Invention
The present invention is directed to resin compositions which are extremely useful in the production of paper products and which provide excellent dry strength properties to paper and paperboard. More particularly, the present invention is directed to resin compositions comprising water-soluble polymers containing a polyhydric alcohol such as sorbitol in the backbone of the polymeric molecule. It has been found that such water-soluble polymers improve the degree of bonding between paper fibers and accordingly increase the dry and the wet strength properties of paper and paperboard.
The water-soluble polymers of this invention containing a polyhydric alcohol such as sorbitol in the backbone of the polymeric molecule are preferably those wherein the polymer backbone comprises polyhydric alcohols and an alkylamine such as diamines or triamines, diacids, glyoxal, citric acid or formaldehyde.
In addition to sorbitol, polyhydric alcohols having from 2 to 6 carbon atoms may be employed as a monomeric polyol, including mannitol and ethylene glycol.
More specifically, the present invention is directed to a resin composition containing from about 30% to about 90% by weight of a polyhydric alcohol, copolymerized with from about 10% to about 70% by weight of a co-monomer. The preferred co-monomers copolymerized with a polyhydric alcohol such as sorbitol include citric acid, a diacid, glyoxal, formaldehyde, and an alkylamine such as diamine or triamine.
The resin composition of the present invention may be prepared by conventional condensation polymerization techniques. Resin slurries thereof may be prepared by diluting the resin solutions with water to a resin concentration of typically less than about 1% by weight. The dilute solution is added to the pulp slurry in an amount to provide from about 0.1% to about 5% by weight of the resin based on dry paper fiber. The pH of the solution can be reduced by the addition of a mineral or organic acid to less than about 3 to increase the reactivity of the resin, if desired.
To realize the maximum potential of the resin composition of this invention, control of the process variables is important. For example, best results are obtained when the resin compositions are added to the paper pulp at the wet-end during the papermaking process. However, the resin compositions may also be added to the paper pulp at the size press.